Sealing of metal bars in carbonized blocks

ABSTRACT

A method for liquid metal sealing of a metal bar in the groove of a carbonized block which includes the application of compressive stresses having an intensity at least equal to the resistance at break under traction of the block material to parts of the block adjacent the groove from a time prior to metal pouring to at least the initial portion of the cooling cycle.

11mm States Patent 11 1 Dumas et al. Dec. 3, 11974 SEALING 0F METAL BARS 1N 3,675,889 7/1972 Peck 249/82 CARBONIZED BLOCKS [76] Inventors: Daniel Dumas, 6 route dI-leyrieux, P E F S H Saint Priest; Georges Hoynant, 145 "W xam".ler rancls usar Chemin de Choulans; Jean Vane, Asszslam Exammer.lohn E. Roethel 33 rue Saint-Jean, both of Lyon, all of France [22] Filed: Mar. 27, 1973 ['57] ABSTRACT [21] Appl. No.2 345,311.

' A method for liquid metal sealing of a metal bar in the 52 us. c1 29/493, 29/195, 29/503 groove of a carbonized block which includes the pp 51 Int. Cl. B23k 1/20 cation of compressive stresses having an intensity at [58] Field f Se h 29/488, 493, 503 195 C; least equal to the resistance at break under traction of 164/100, 103, 121; 249/82 the block material to parts of the block adjacent the groove from a time prior to metal pouring to at least 5 References Cited the initial portion of the cooling cycle.

UNITED STATES PATENTS 3,286,973 ll/l966 Sylvester 249/82 8 Claims, 2 Drawing Figures This invention relates to an improvement in carbon blocks having metal bars sealed therein, such as employed as crucible bottoms in electrolytic production of aluminum.

The bottom of the crucibles of industrial baths of the type used for the electrolytic production of aluminum are generally formed of several pre-baked anthracite or graphite based carbon blocks, or blocks of other carbonized materials arranged in juxtaposition. Referring to FIG. 1, each carbon block 1, whichis to be used as a cathode current supply for electrolysis, is equipped with a metal current feed bar 2 retained in an elongate groove 3, formed to extend substantially throughout the length of the block. The portions located on opposite sides of the grooves 3 are called wings of the block. t

In order to fix the bar in the groove sufficient to provide' electrical contact between the bar and the block,

two techniques have been employed. A carbon paste is either tamped into the open space about the bar in the .block, or a molten metal is poured into the open space to fill the space between the bar and the block.

The method of pouring liquid metal can be adapted to mass production, with minimum investment, but it has the objection of subjecting the carbon blocks to such violent thermal shock as often to cause the appearance of transverse cracks 4, which adversely affect the outer ridges 6 of the wings 5 of the block. Such cracks do not usually appear in blocks of short length but theybecome more frequent, with increased seriousness, in blocks of greater length. 7

The conventional sealing practice is as follows:

The cathode blocks and the current supply bars, generally of mild steel, are preheated to a temperature of the order of 400700 "Cfin the center portion of the bar and of theorder of 250450C in the portion of the block adjacent to the groove. I

Preheating is followed by the casting of the liquid metal, generally castiron, at a temperature within the range of l,250-l ,500C. Under the described preheat-- ing conditions, it is possible usually to pour the cast iron to fill the entire free space about the bar, within the groove.

During the time immediately following pouring, the front sides of the grooves are raised to a rather high temperature, which is quite close to that of the cast iron, while very little of the heat penetrates into the interior of the block, and the outer portions of the block are still at preheating temperature or slightly below due to natural cooling.

Thus, significant heat gradients, and as a result considerable heat expansion gradients exist throughout the cross section of the block. These. produce mechanical stresses which may lead to visible localized breaks in the vicinity of the outer ridges of the block in the form of cracks, the surface of which is usually perpendicular to the direction of the ridge. These cracks appear shortly after casting, generally within 10 minutes. While very plainly visible at the time of their formation, they. close again as the block cools and becomes isothermal. However, the discontinuity of the material prevails adversely to affect the mechanical properties of the cathode assembly.

Thus it is an object of this invention to provide a .mearis. and method for producing blocks of the type described without the described damaging consequences of heat shock.

These and other objects and advantages of this invention will hereinafter appear and for purposes of. illustration, but not of limitation, an embodimentof the invention is shown in the accompanying drawing, in which FIG. 1 is a perspective view of a carbon block and bar assembly of the type to which this invention is addressed, and

FIG. 2 is a perspective view similar to that of FIG. 1 showing the application of compressive forces in accordance with the practice of this invention.

The improvement embodying the features of this invention applied to the customary method of sealing by molten metal includes means which avoid the formation of cracks and other inconveniences characteristic of prior production.

In accordance with the practice of this invention, the sealing method which includes the steps of applying a longitudinal groove to the carbonized block, arranging the bar in said groove, preheating the block and bar,

and pouring the liquid metal into the groove around the bar is characterized by the further application of a longitudinal compressive stress of an intensity at least equal to the resistance at break in traction of the block material to at least part of the block adjacent to the outer longitudinal ridges of the wing, said stress being applied prior to metalpouring and maintained at least during the initial part of the cooling cycle. In the case of sealing with cast iron and natural cooling, the stress is maintained for at least 5 minutes following metal pouring.

In order to apply the stress, a compressive force (F) is applied parallel with the longitudinal ridges of the block, as illustrated in FIG. 2. Such force can be applied by various means, such for example as by flanges, screw jacks, and the like exerted on at least a portion 7 of the end surfaces of the wings 5 of the blocks 1. The zones of application 7 of these forces are preferably adjacent to the-ends of the outer ridges 6.

The resistance at break in traction of the carbonized blocks used, whether-formed of graphite or a graphite or anthracite base, ranges between 10 and 50 kgflcm It is sufficient to apply a total compressive force to the ends of the wings of the block, such that the resulting stress will be, at each point of the involved zone of the block, higher than this resistance under traction.

,Thus, the method consists of prestressing to precompress the zones of the blocks which are degraded most by the heat shock resulting from pouring of the sealing metal, during the entire duration necessary for dissipation of the heat shock. The method of the invention applies to all types of carbonized cathode blocks. It not only is free of inconveniences of its own but it reduces cost and offers improvements in production at the operating station. Finally, it allows for reduction of the preheating time and temperatures.

The following examples show the advantages of the method of the invention. All of the examples make use of cathode blocks based on calcined anthracite with a resistance against traction of 33 kgf/cm The blocks are 310 cm long and have a cross section of 50 X 45 cm. The groove is 15 cm deep and 17 cm wide at its widest portion. The bar is 14 cm in height and 12 cm in width. Under these conditions, both wings are approximately 16 cm in width.

In all of the tests, the temperatures were measured in the median cross section of the blocks. At the top of the blocks, the temperature was taken in the wings, as near as possible to the groove. At the bottom, the temperature was taken below the blocks.

Example l The sealing was carried out according to the usual method, without prestress. The preheating time was 4 hours for all blocks. The bars were preheated to temperatures ranging from 580 to 650C, the tops of the blocks were at 400-450C, the bottoms of the blocks at l150C. The cast iron was poured at a temperature between l,380 and l,450C. It was noted that 80 percent of the blocks so prepared presented at least one transverse crack on one wing.

Example 2 Preheating for 2 hours.

Preheating temperatures:

bars 450500C tops of blocks 280300C bottoms of blocks 5060C.

The cast iron was poured, without application of prestress, at a temperature between l,380 and 1,430C. It was noted that all blocks had at least one transverse crack on one wing and 75 percent of them had at least three large cracks.

Example 3 The preheating and temperature conditions of the cast iron were the same as in Example 2. However, in addition, a total force of 3 tons was applied to each wing of the blocks, in longitudinal compression, by means of screw jacks bearing onto plates of 10 X l0 cm placed on the front ends of the wings, in the vicinity of the outer ridges, according to the diagram in FlG. 2. This prestress of 30 kgflcm slightly below the resistance under traction of the block material, was maintained for 10 minutes after the pouring. 30 percent of the blocks presented at least one transverse crack on one wing.

Example 4 The preheating and temperature conditions of the cast iron were the same as in Example 2. The total compression force, applied to each wing like in Example 2, was 5 tons, corresponding to a prestress level of 50 kgf/cm. This prestress was maintained for minutes after the pouring. Not one. block had any wing crack.

Example 5 Heating time: 1 hour Preheating temperatures:

bars 400.470C

tops of blocks 200220C bottoms of blocks 3040C Temperature of the cast iron: l,380 to 1,430C.

If no prestress is applied, all the blocks present at least three important wing cracks.

If a compression force of 5 tons per wing is applied on plates of 10 X 10 cm, and if it is maintained for l0 minutes after the pouring, none of the blocks have any wing crack.

Example 6 Preheating and temperature conditions of the cast iron were identical with those of Example 5.

The prestress force was 5 tons per wing on plates of 10 X 10 cm. It was maintained for 5 minutes after the pouring. It was noted that 10 percent of the blocks presented at least one transverse crack on one wing, which appeared at the time when the prestress was eliminated.

A comparison of Examples l to 4 shows the influence of the prestress and its total efficiency when its intensity exceeds the resistance under traction of the block material.

A comparison of Examples 2 and 4 and that of the two cases of Example 5 shows the possibility of reducing the preheating temperatures by operating according to the method of the invention.

A comparison of Examples 5 and 6 shows the influence of the time during which the prestress is maintained during the cooling.

The method of the invention, while advantageous in all cases, is not limited to any manner of preheating nor of the carbonized material constituting the block.

It will be apparent from the foregoing that there is provided a simple and efficient means for eliminating the formation of cracks due to heat shock in the manufacture of carbonized blocks with metal bars of the type described.

It will be understood that changes may be made in the details of conditions and operations without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. A method for producing a carbonized block having a metal bar sealed therein for establishing good electrical contact therebetween including the steps of providing the block with a longitudinal groove in one surface thereof, arranging the bar in said groove, preheating the block and bar to elevated temperature, and pouring liquid metal into the groove around the bar, applying a longitudinal compressive stress of an intensity at least equal to the resistance at break under traction of the block material to at least portions of the block adjacent the groove, said compressive stress being applied prior to metal pouring and maintained during at least an initial portion of the cooling time.

2. A method as claimed in claim 1 in which the portions adjacent the groove comprise wing portions and in which the compressive stress is applied to the block adjacent the outer longitudinal ridges of the wing.

3. A method as claimed in claim 2 in which the compressive stresses are applied to a portion of at least the front end of the wings of the block.

4. A method as claimed in claim 2 in which the compressive stress is applied in the vicinity of the outer ridges of the wing.

5. A method as claimed in claim 1 in which the sealing metal is cast iron poured at a temperature within the range of l,380-l,450C.

6. A method as claimed in claim 1 which includes the step of allowing the poured metal to cool naturally and in which the compressive stress is maintained for more than 5 minutes after metal pouring.

7. A method as claimed in claim 1 in which the bar is preheated to a temperature within the range of 400500C, and the block is preheated to a temperature within the range of 200-300C at the top and 30-60C at the bottom.

8. A carbonized block for use as a cathode of an aluminum electrolysis cell with the current feed bar sealed therein by the method of claim 1.

UNITED STATES PATENT OFFICE CERTIFICATE OF CGRRECTION Patent No. 3,851,377 Dated December 3, 1974 I Daniel Dumas et al It is certified that error aooears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Assignee: Societe Des Electrodes Et Refractaires "SAVOIE" Paris, Frar lce Signed and sealed this 1st day of April 1975.

(SEAL) Attest:

C. ZMRSHALL DJiNN RU H C. Z-ZASON Commissioner of Patents attesting Officer: and Trademarks 

1. A method for producing a carbonized block having a metal bar sealed therein for establishing good electrical contact therebetween including the steps of providing the block with a longitudinal groove in one surface thereof, arranging the bar in said groove, preheating the block and bar to elevated temperature, and pouring liquid metal into the groove around the bar, applying a longitudinal compressive stress of an intensity at least equal to the resistance at break under traction of the block material to at least portions of the block adjacent the groove, said compressive stress being applied prior to metal pouring and maintained during at least an initial portion of the cooling time.
 2. A method as claimed in claim 1 in which the portions adjacent the groove comprise wing portions and in which the compressive stress is applied to the block adjacent the outer longitudinal ridges of the wing.
 3. A method as claimed in claim 2 in which the compressive stresses are applied to a portion of at least the front end of the wings of the block.
 4. A method as claimed in claim 2 in which the compressive stress is applied in the vicinity of the outer ridges of the wing.
 5. A method as claimed in claim 1 in which the sealing metal is cast iron poured at a temperature within the range of 1,380*-1, 450*C.
 6. A method as claimed in claim 1 which includes the step of allowing the poured metal to cool naturally and in which the compressive stress is maintained for more than 5 minutes after metal pouring.
 7. A method as claimed in claim 1 in which the bar is preheated to a temperature within the range of 400*-500*C, and the block is preheated to a temperature within the range of 200*-300*C at the top and 30*-60*C at the bottom.
 8. A carbonized block for use as a cathode of an aluminum electrolysis cell with the current feed bar sealed therein by the method of claim
 1. 